1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly to an optical pickup device suitable for a compatible optical pickup device capable of emitting several kinds of laser light beams having different wavelengths to a recording medium.
2. Description of the Related Art
Currently, various optical discs such as a compact disc (CD) and a digital versatile disc (DVD) have been commercialized and widely used. Recently, next-generation DVD standardization for recording and reproducing information using a blue-violet laser light beam has been proceeded. In the next-generation DVD, information is recorded and reproduced using the blue-violet laser light beam having a wavelength of about 405 nm. When the wavelength of the laser light beam shortens, a higher density can be obtained.
Therefore, when the variety of optical discs increases, development of a so-called compatible optical pickup device capable of performing recording and reproduction on different kinds of optical discs is desired. In order to irradiate an optical disc with laser light beams having different wavelengths, it is possible to employ an arrangement in which semiconductor lasers that emit the laser light beams having the respective wavelengths are separately located. However, when such the arrangement is employed, spaces for locating the respective semiconductor lasers and optical elements for guiding the respective laser light beams to an objective lens are separately required, with the result that increases in outer size and the number of parts occur. Thus, an arrangement in which a plurality of laser elements having different emitting wavelengths are provided all together in a single CAN package has been studied. According to such the arrangement, a space for locating the semiconductor lasers can be reduced and an optical system can be commonly used among the respective laser beams.
However, when the plurality of laser elements are provided in the single CAN package as described above, a deviation occurs between the optical axes of the laser light beams according to arrangement gap between the respective laser elements. Therefore, when the optical axis of the optical system is aligned with the optical axis of a laser light beam, the optical axes of other laser light beams are deviated from the optical axis of the optical system. Thus, in the case of recording and reproduction using the other laser light beams, there arises a problem in that aberration of laser light beams is produced on an optical disc or a photo detector to cause deterioration of optical characteristics.
In order to solve such the problem, according to a conventional art described in JP 06-131688 A, a birefringence element is disposed immediately after a semiconductor laser including several kinds of laser elements, and the optical axes of the respective laser light beams are aligned with one another by the birefringence element to guide the laser light beams to the optical system. That is, the plurality of laser elements are disposed in the same CAN package such that the polarization plane of a reference laser light beam is orthogonal to the polarization plane of each of other laser light beams. The birefringence element that transmits the reference laser light beam and refracts the other laser light beams such that the optical axes thereof are aligned with the optical axis of the reference laser light beam is disposed immediately after the semiconductor laser. According to the structure described above, the laser light beams can be guided to the optical system located in the subsequent stage after the optical axes of the respective laser light beams are aligned with one another. Thus, the laser light beams having the respective wavelengths can be converged to a recording medium without aberration.
According to a technique described in JP 11-134702 A, a diffraction grating is disposed immediately before a photo detector that receives reflected light beams from an optical disc, thereby guiding reflected light beams having respective wavelengths to the photo detector. That is, three laser elements are disposed in the same CAN package. Laser light beams having different wavelengths which are emitted from the respective laser elements are converged onto the disc by the same optical system. Reflected light beams from the disc are diffracted by the diffraction grating and converged onto the photo detector. According to the structure described above, the respective laser light beams can be adequately converged to the photo detector. Therefore, it is possible to obtain a detection signal with no fluctuation.
The conventional art described in JP 06-131688 A requires an additional birefringence element. In addition, it is necessary to form the respective laser elements such that the polarization plane of the reference laser light beam is orthogonal to the polarization plane of each of the other laser light beams. However, it is hard to form the laser elements in which the polarization planes of the laser light beams are different from one another. When the additional birefringence element which is expensive is disposed, a problem in that a cost of the entire optical pickup device increases also occurs.
Refracting action of the birefringence element depends on the frequency. However, in view of the wavelengths of the laser light beams used for the compatible optical pickup device, a refraction angle when each of the laser light beams having the respective wavelengths is refracted by the birefringence element is not significantly changed. For example, a wavelength difference between a laser light beam for CD (780 nm in wavelength) and a laser light beam for DVD (655 nm in wavelength) is only about 100 nm. As a result, refraction angles of both the laser light beams which are produced by the birefringence element become substantially equal to each other.
Therefore, when the optical axis of the laser light beam for CD and the optical axis of the laser light beam for DVD are to be aligned with that of a laser light beam for next-generation DVD by the refracting action of the birefringence element, it is necessary to allow the laser light beam for CD and the laser light beam for DVD to enter the birefringence element with a state in which their optical axes are approximated to each other to such a degree that these axes are substantially aligned with each other. However, it is nearly impossible to dispose the laser elements with the state in which their optical axes are approximated to each other to the degree in manufacturing. Thus, it is very hard to align the optical axis of the laser light beam for CD (780 nm in wavelength) and the optical axis of the laser light beam for DVD (655 nm in wavelength) with that of the laser light beam for next-generation DVD by the birefringence element.
According to the conventional art described in JP 11-134702 A, the reflected light beams having the respective wavelengths are subjected to diffracting action by the diffraction grating disposed immediately before the photo detector, thereby guiding the respective light beams onto the photo detector. Therefore, when a variation in distance is caused between the optical axes of the reflected light beams having the respective wavelengths, the reflected light beams having the respective wavelengths cannot be adequately guided onto the photo detector unless a design of the diffraction grating is suitably modified according to the changed distance.
However, every time a variation in arrangement gap between the laser elements is caused due to a manufacturing error or the like, it is impractical to redesign the diffraction grating according to the arrangement gap. Therefore, in practice, there is no option but to use an existing diffraction grating without any modification in such a case, with the result that optical axis correction cannot be adequately performed. When a design value of an arrangement gap between laser elements is changed among makers, it is necessary to separately prepare respective diffraction gratings for the makers.